In communication systems based on e.g. Code Divisional Multiple Access (CDMA), outer loop power control is used to meet the desired quality of service targets. The outer loop power control is implemented both in the user equipment to meet the downlink quality target and also in the base station to meet the uplink quality target. In Cellular Networks, the downlink is the transmission path from the base station to the user equipment, and the uplink is the transmission path from the user equipment to the base station. It is important that the outer loop power control is able to maintain the desired quality of service target despite varying radio conditions, which is often the case in cellular systems.
The following describes various technical aspects related to inner loop power control, outer loop power control and its convergence in CDMA systems. Some specific details are related to Wideband Code Division Multiple Access (WCDMA) but are equally applicable to other CDMA based technologies such as e.g. cdma2000 because power control, both inner and outer loop, is the hallmark of CDMA access technology. The methods may also be implemented in a Fraction High Speed Downlink Packet Data Access (F-HSDPA).
In CDMA systems the inner loop power control, also called fast power control, runs every time slot, which is typically less than 1 ms (e.g. 0.67 ms in WCDMA). In WCDMA the inner loop power control runs in both uplink and downlink. The aim of the uplink and downlink inner loop power controls is to counter the effect of fast fading, while maintaining the desired Signal to Interference and noise Ratio (SIR) target. In the uplink it also ensures to compensate for the near-far problem, so that the signal received from the users far out in the cell are not swamped out by the stronger signal. During every slot the User Equipment estimates the SIR on some known reference or pilot symbols and compares it with some SIR target corresponding to the given service (e.g. certain BLock Error Rate (BLER) requirements, spreading factor used etc.). In WCDMA, Downlink SIR is measured on Dedicated Physical Control Channel (DPCCH), which comprises pilots and Transmitter Power Control (TPC) commands for uplink power control. If the measured SIR is less than the SIR target then the user equipment generates UP command, otherwise it generates DOWN command; in response the base station will increase (in case of UP) or decrease (in case of DOWN) its downlink transmit power.
The aim of outer loop power control is to maintain a certain link quality in terms of Frame Error Ratio (FER), BLock Error Ratio (BLER), Bit Error Rate (BER) or any other suitable measure such as outage probability. The quality target is the ultimate quality target measure, which is set by the network and is expected from the user equipment to consistently maintain this target to ensure the desired quality of service is met throughout the call session. The value of the quality target depends upon the type of service, which in turn impacts the SIR target used for inner loop power control, as explained above. Typically 1% BLER target is used for speech, 10% BLER target is used for packet data, 0.1 BLER % is used for video telephony and so on. Due to the varying radio link conditions e.g. user mobility, fast fading etc, the mapping between the SIR target and BLER changes over time. This is a key point as it requires constant adjustment of the SIR target to maintain the desired value of BLER. This mechanism of adjusting the target SIR is also outer loop power control or quality control.
The most commonly used algorithm used to run outer loop power control is the jump algorithm as depicted below:
      S    ⁢                  ⁢    I    ⁢                  ⁢                  R        t            ⁡              (                  k          +          1                )              =      {                                                      S              ⁢                                                          ⁢              I              ⁢                                                          ⁢                                                R                  t                                ⁡                                  (                  k                  )                                                      +                          S              ⁢                                                          ⁢              I              ⁢                                                          ⁢                              R                s                                                                          if            ⁢                                                  ⁢            block            ⁢                                                  ⁢            is            ⁢                                                  ⁢            erroneous                                                                          S              ⁢                                                          ⁢              I              ⁢                                                          ⁢                                                R                  t                                ⁡                                  (                  k                  )                                                      -                                                            B                  ⁢                                                                          ⁢                  L                  ⁢                                                                          ⁢                  E                  ⁢                                                                          ⁢                                      R                    t                                                                    1                  -                                      B                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    E                    ⁢                                                                                  ⁢                                          R                      t                                                                                  ⁢              S              ⁢                                                          ⁢              I              ⁢                                                          ⁢                              R                s                                                                          if            ⁢                                                  ⁢            block            ⁢                                                  ⁢            is            ⁢                                                  ⁢            correct                              
SIRt denotes the SIR target, BLERt is the target block error ratio and SIRs is the step by which the target SIR is increased in each iteration. The SIRs is implementation dependent and may typically be 0.5 dB or 1 dB per transport block received.
An important observation is that the increase in SIR target in response to an erroneous block is much larger than the decrease in the SIR target when the block is correctly received. Indeed, the decrease in the SIR target is linked to the BLER target, which is set by the network.
The algorithm is applied to every received transport block in a transport channel after every Transmission Time Interval (TTI), which is typically 20 ms for speech but generally shorter, e.g. 10 ms for packet data. This means that the SIR target is adjusted at least once per TTI. Secondly if there is more than one transport block per TTI per transport channel, which is often the case with services other than speech, the SIR target will be adjusted several times per TTI, i.e. number of times the transport blocks per TTI.
In practice every radio connection or simply radio access bearer comprises several transport channels: at least one Data Transport CHannel (e.g. DTCH) and more than one control signalling channels (e.g. 2 or more Dedicated Control CHannels (DCCH)). Thus, an important characteristic of the outer loop power control is that it should run for each individual transport channel. The SIR target for the inner loop power control, i.e. the final SIR target since there is only one inner loop power control, should be derived from the SIR target obtained from the outer loop power controls of all the transport channels in the radio access bearer. More specifically the SIR target for the inner loop power control should be the maximum of the SIR target values used for the multiple outer loops as expressed in the equation below for N transport channels per radio access bearer:SIRt—innerloop=max(SIRt—OLt,SIRt—OL2,Λ,SIRt—OLN)
The main advantage of the outer loop power control algorithm is its robustness and implementation simplicity. But the major limitation is that it is inherently slow because SIR target is changed on TTI level, which is typically in the order of 10 to 20 ms. This should be compared with the situation in the inner loop power control, which runs every time slot, typically less than 1 ms.
The BLER convergence time depends upon the occurrence of block errors events. At low BLER target, e.g. 0.1%, the mean time between the block errors is considerably large. Also, the use of a reference channel with multiple transport blocks per transport channel reduces the average time between the block errors by a factor of the number of blocks. This in turn decreases the BLER convergence time to some extent.
When BLER does not converge or converges too slowly, the main problem is that the user equipment will drive excessive and unnecessary downlink transmitted power resource from the base station. The transmit power is a rare resource, whose inefficient utilisation leads to loss in system capacity.
Different BLER target values will influence the outer loop power control convergence performance. Apparently the initial convergence is the major problem since the initial radio conditions are not well known. However, slow convergence or in some cases unstable outer loop power control behaviour can also occur if there is an abrupt change in the radio conditions. For instance when user equipment is moved from bad to good radio conditions, the SIR target in the latter state (good condition) will remain too high for a considerable amount of time, e.g. few seconds, due to the inherently slow reactive behaviour of the outer loop power control. The overall impact will result in draining the base station radio resources, notably the transmitted power, leading to system throughput loss. This means some specific solution is needed when these type of phenomena occur. The opposite scenario, where user equipment is moved from good radio conditions to the bad radio conditions is less stringent. Since in this case the outer loop power control will quickly lower the SIR target, thereby speed up the convergence.
The jump algorithm used in the outer loop power control may also lead to windup effect especially under heavy load, deep and long shadow fading or in situations when the user equipment is in the cell border region. The windup effect refers to the case when the SIR target increases or decreases indefinitely in one direction. Eventually this will lead to a situation when the downlink transmitted power either hit the maximum transmit power or minimum transmit power values allocated for each channel. This is an unstable behaviour and must therefore be avoided. Therefore, anti-windup protection is implemented in terminals.
The aim is to suspend the outer loop power control, i.e. not to change the SIR target, provided the condition |SIRm−SIRt|≧Γ is satisfied SIRm denotes the estimated value of mean SIR generally measured every frame (10 ms); Γ is a threshold margin expressed in dB and is typically 2-4 dB. After the windup situation has vanished the user equipment generally resumes the outer using the last SIR target value, i.e. the value used until the suspension of outer loop. The main problem with the current approach is that it does not address the scenario where radio conditions would change in case the user equipment remains in windup for a long duration.
The convergence of the current outer loop power control is inherently slow as explained above. This means that in case of abrupt changes in the radio signal conditions the outer loop power control reacts with the same pace. Due to slow convergence the system performance in terms of downlink transmit power, is degraded i.e. requires more downlink power in the following scenarios:                During initial convergence of outer loop power control;        During convergence of outer loop power control when radio signal condition changes abruptly,        At present anti-windup, which does not adapt well to fast varying conditions;        During convergence of outer loop power control just after the windup effect.        
The adverse performance of the currently used outer loop power control in these scenarios has been discussed within the 3rd Generation Partnership Project (3GPP) whereby state of the art solutions for solving these problems are discussed below.
The initial convergence of outer loop power control can be improved by setting an appropriate initial SIR target at the start of the call or after long inactivity periods. The user equipment specification does not provide any test case that tests the initial convergence of BLER, i.e. the convergence at the start of the call within a certain time. The current requirements on BLER convergence are tested in steady state conditions. A prior-art solution to speed up the initial convergence includes e.g. the following features:                The SIR target is set as the sum of two terms: SIR outage target and SIR BLER target.        The Initial SIR BLER target is the mean of the SIR BLER target used during at least two previous transmissions.        The initial SIR outage target is the mean of the SIR outage target but is slightly higher than the target used in the previous transmission. In addition the initial SIR outage target can also be a value between 80th and 95th percentiles or simply 90th percentile.        
However, the above solution has at least two disadvantages: The above solution requires that the network provides two quality targets, target BLER and target outage probability. This will lead to more complexity in the system since the user equipment has to handle two different quality measures to estimate the correct SIR target. It also leads to increased signalling overheads in the downlink due to additional quality target. Another disadvantage is that the initial SIR target setting relies on the averaging of SIR target used in the previous transmissions. At the time of initial SIR target setting either the previous SIR target is not available or it may not be reliable as the radio signalling conditions may have changed.
Further, it has been observed that outer loop power control convergence is slow after an abrupt change in the radio signal conditions. More specifically, when a user equipment moves from bad radio conditions to good radio conditions, the SIR target decreases very slowly i.e. when the user equipment is in good radio conditions. Since the SIR target remains considerably higher than desired, the main impact is that the downlink transmit power will remain high. The specification does not comprise any test cases to ensure functionality in the user equipment that could handle outer loop power control convergence in such a scenario.
It is shown in document EP 1575185 A1 that outage based outer loop power control that relies on a new quality target, outage probability, leads to faster convergence of the outer loop power control. The problem with this solution is that it works in an iterative fashion, based on the Newton-Raphson method requiring intense processing, which eventually drains the user equipment power consumption. It should be noted that each service or connection requires several parallel radio bearers and outer loop power control runs on each radio bearer. Therefore aggregate processing will be significant resulting in much higher user equipment power consumption. Another important aspect is that abrupt changes in radio conditions may not occur extremely frequently, e.g. once every 10-20 seconds depending upon environment. Therefore it is sufficient to have some special mechanism to tackle this situation. It is therefore not appropriate to completely change the currently used and well established outer loop power control methodology to solve one particular issue.
When a windup situation is detected, an anti-windup algorithm is activated to prevent an indefinite change in the SIR target in one direction. In other words, the outer loop power control is suspended until the end of the windup situation. The duration of the windup can vary depending on the fading and shadowing effects. When the windup situation is finished the user equipment restarts with the last value of the SIR target. However, one major problem is that during the windup the radio conditions may change considerably and therefore the old SIR may not be valid any more.